The invention comprises a method and apparatus for continuously controlling the caustic soda content of a cake for trihydrate of alumina, Al(OH).sub.3, travelling on a conveyor belt located at the outlet from a rotary drum filter. More generally, the method and apparatus apply to continuously controlling the content of an impurity which can have a colour reaction with a reagent, in a product travelling on a conveyor belt in the form of irregular lumps.
In the Bayer method of preparing alumina by reacting bauxite with caustic soda (FIG. 1), the aluminate liquor obtained is freed from its insoluble impurities in the caustic soda and passes into a series of reactors known as decomposers (1). In the decomposers part of the aluminate in solution is precipitated in the form of insoluble hydrate of alumina, by seeding with germs of hydrate of alumina. The hydrate of alumina is then filtered through a rotary filter (2). The cake obtained naturally contains a large amount of dissolved caustic soda, particularly in the interstitial liquid of the cake. In order to reduce the quantity of caustic soda the process is carried out in two stages:
the cake of hydrate of alumina is put back into suspension in a tank (3) fitted with an agitator, in the washing water from a filter (4) referred to below, to form an alumina milk.
the suspension thus obtained is filtered through a drum filter (4) under vacuum; the cake formed is washed with hot water in the same filter. The washing liquor is passed through the pipe (5) to the tank (3), where it will be used to put the hydrate of alumina into suspension.
The cake obtained on the drum filter (4) after washing is removed from the filter and drops onto a conveyor belt (6) which takes it to the calcining furnace.
For various reasons the free caustic soda content of the hydrate of alumina leaving the drum (4) and travelling towards the calcining station has to be kept below a certain limit, 300 ppm, and it is not essential to go below 175 ppm. The free caustic soda can be determined very easily by acidimetry on samples which are taken from the conveyor belt (6) from time to time. But this is quite a long process: the sample has to be dried in an oven for 3 hours, then extracted with boiling distilled water for 30 minutes, and titrated in the presence of a coloured indicator or a pH meter. Now the throughput of alumina on the conveyor belt is several tens of tonnes per hour; from the time when the sample is taken to the time when the result is obtained, a considerable tonnage of possibly non-standard alumina may be conveyed to the calcining station. In order to avoid too much variation in the product, a worker equipped with a pipette of a phenolphthalein solution is posted near the conveyor belt. By projecting the solution onto the alumina travelling in front of him on the belt, he can ensure--very roughly of course and by estimating the shade of red obtained--that there is no serious variation in the caustic soda content of the product.